Semiconductor laser diodes are used in many applications where compact size and/or high efficiency is important. Semiconductor laser diodes offer relatively low cost, high reliability and simplicity of use.
Single emitter multi-mode laser diodes are commonly available in various wavelengths with radiation power output up to 2 Watts or more. These lasers typically have a rectangular or stripe emitter around 1 μm high and in the range of approximately 20 μm-500 μm long. Fundamental problems of heat removal and optical emitter facet damage limit the power that can be emitted per unit length of emitter without significantly reducing the operating lifetime of such laser diodes.
For applications requiring more than a few Watts of radiation power it is common to use an array of single emitter diodes. One could form such an array by mounting single emitter diodes on a mechanical support. It is more common to fabricate the array of emitters on a monolithic substrate. These devices, known as laser diode bars, are available in many configurations with radiation power of up to 50 Watts.
A monolithic laser diode array 1 is shown in FIG. 1. It has a semiconductor substrate 2 which carries an array of emitters 3. Adjacent emitters have a dead space between them that does not emit light. Due to emitter geometry, radiation beam 4 is substantially asymmetrical and has differing divergence rates in the x-axis and y-axis directions. The full-width divergence in the y-axis is typically in the range of 40° to 100° and in the x-axis, 8° to 20°. The y-axis may be referred to as the “fast” axis while the x-axis may be referred to as the “slow” axis. The high beam divergence of semiconductor diode lasers makes it necessary to collimate or focus the beams emitted by such lasers for most applications.
The beam quality in the y-axis can be very good, with an M2 value of close to 1.0. M2 is a dimensionless parameter that characterizes the degree of imperfection of a laser beam. An ideal, diffraction-limited, Gaussian profile beam would have an M2 of 1.0. Any departure from the ideal results in an M2 value of greater than 1.0. The M2 of the beam from a laser diode in the x-axis is very poor, signifying a substantial deviation from a perfect beam. This difference in the beam quality, along with the differing divergence rates for the x and y axes, make it necessary to treat the axes separately when designing a collimation scheme.
Spatial light modulators offer an advantage in imaging in that they can be fabricated as multi-channel devices, thus reducing system complexity while increasing imaging speed. Spatial light modulators are optical modulators constructed to spatially modulate, according to prescribed input, a readout optical beam. Spatial light modulators having a single line of modulating elements or areas are of particular use in imaging tasks although in some applications multi-line devices can also be advantageous. Examples of spatial light modulators include a wide range of electro-optical, acousto-optical, and electromechanical devices.
While laser diode bars have several advantages for illuminating a spatial light modulator one must first overcome the challenges set the by format of the laser diode beam. For optimal illumination of a line spatial light modulator, the laser bar radiation must be precisely transformed into a line of uniform illumination in a manner that maximizes brightness. Brightness is defined as the luminous flux emitted from a surface per unit solid angle per unit of area.
U.S. Pat. No. 5,517,359, to Gelbart discloses a method of formatting the output from a laser diode to form a line source which is particularly useful for illuminating a spatial light modulator. Radiation from each emitter of the laser diode is fully overlapped at the modulator in both the x and y axes. A cylindrical microlens collimates the radiation in the y-axis. In the x-axis an array of cylindrical microlens elements collimate and steer the radiation towards a common target point, some distance from the laser, overlapping the radiation profiles.
The overlapping of emitter radiation profiles is advantageous should one or more emitters fail. Since the overall profile is the sum of a plurality of emitters, an emitter failure only reduces power and does not substantially change the profile. In contrast, if only the fast axis is collimated and the slow axis is allowed to diverge up to a point where the beams overlap only partially, an emitter failure will severely compromise the profile. Another advantage of overlapping is that dead space between emitters is effectively removed, creating a high brightness illumination line.
A problem that occurs in using many laser diodes bars is that, as a result of stress-induced bending of the device wafer, the emitters are not in a perfectly straight line; a characteristic known as “smile”. While bars have been manufactured with sub-micron smile, it is more common to have to deal with around 5-10 μm of smile. A non-negligible smile prevents precisely aligning the beams in the y-axis and thus degrades line quality.
U.S. Pat. No. 5,861,992 to Gelbart provides an individual microlens in front of each emitter. The microlens is adjusted in the y-axis direction to line up all emitter radiation profiles at a target plane. The microlenses collimate radiation from the emitters in both axes and additionally can be used to steer the emitter profiles to overlap in the x-axis direction. The microlenses are individually sliced from the center of a molded aspheric lens, such that each slice is substantially the same as the diode array pitch.
Advances in semiconductor materials have lead to the available power from laser diodes bars more than doubling over the past few years. However, despite these advances, it is unlikely that there will be a further doubling of power levels in the near future unless there is a significant breakthrough in the art. On the other hand, applications continue to demand higher overall laser powers.
U.S. Pat. No. 4,716,568 discloses a plurality of linear diode laser array subassemblies stacked one above the other and simultaneously powered from a single source. In this configuration, power can easily be scaled by simply adding more laser diode arrays. The downside is that it is very difficult to design combination systems that deal with the radiation asymmetry while simultaneously preserving brightness for a vertical stack. While this combination scheme is effective at increasing the power available, the loss of brightness counters much of the gain, particularly for demanding imaging applications.
U.S. Pat. No. 6,240,116 discloses a stepped reflector that can be used to combine beams from multiple laser diodes, simultaneously correcting some of the asymmetry while conserving brightness. However the stepped reflector is a complex component to manufacture and adds to system cost and complexity. The stepped reflector does not avoid the need for a separate microlens for each emitter to achieve a good profile.
Accordingly, there is a need for apparatus and methods for combining the beams from two or more laser diode arrays to achieve higher power than is available from a single bar diode. There is a particular need for such methods and apparatus which:                combine radiation in such a way that brightness is maximized;        are simple and cost effective;        preserve the beam quality in the y-axis so that a substantially Gaussian profile is maintained; and,        combine the beams in such a manner that the far field profiles are substantially uniform in the x-axis.        